1. Field of the Invention
This invention is related to the field of manufacture of cooling holes in the blades of high-temperature gas turbines.
2. Description of the Related Art
It is well known from basic principles of thermodynamics that the higher the temperature of combustion gases entering a turbine, the higher the efficiency of the turbine becomes. Therefore, over the years the design of turbines has evolved to include an increase in the gas inlet temperature as one of the changes to improve efficiency and performance. Since higher inlet temperature necessarily results in higher operating temperatures, the strength of the metal turbine blades and their tolerance to high-temperature operation have been a limiting factor to providing further improvements. At steady state operation, the temperature of turbine blades depends on the temperature of the gases flowing through the turbine as well as on heat transfer between the blades and the gases. Therefore, during the last ten years the construction of gas turbine blades has included cooling holes to increase heat transfer from the blade to the surrounding medium. These holes are typically 1.3 to 3 mm in diameter and 20 to 40 cm in length, bored as straight cylindrical apertures (as shown in FIGS. 1a and 1b) across the thickness of the turbine blade by an electrochemical process known in the industry as the Shaped Tube Electrochemical Machining (STEM) drilling process.
In the continuing effort to improve the thermodynamic efficiency of turbines by further increasing the inlet gas temperature, the cooling efficiency of straight holes is no longer sufficient to maintain the temperature of the blades within acceptable limits. Thus, new hole designs have been employed to change the character of the gas flow through the holes in order to increase heat transfer. To that end, the gas turbine industry has recognized for some time the need to produce turbine blades with internal ridges to effect turbulent flow through the holes. These ridges, which are normally called turbulators, thus increase the efficiency of the turbine blade cooling.
An electrochemical method of producing cooling holes with circular ridges (such as shown in FIGS. 2a and 2b) has been developed in the industry, but it involves the use of specially designed electrodes. In addition, if it is desired to maintain the drilling speed available while drilling straight holes, larger power supplies are required than are normally used for drilling holes by the STEM process. These ridges provide an improvement over straight holes, but are not necessarily optimal for all operating conditions. Therefore, it would be advantageous to have a drilling system that permitted the drilling of holes with turbulators placed in a variety of configurations, especially if it could be done with existing equipment. Thus, there exists a need for a method of manufacturing cooling holes with turbulators using standard electrodes or modified electrodes in combination with standard STEM process equipment. This invention is directed as satisfying that need.